Optical film and polarizing beam splitter

Abstract

Optical films and polarizing beam splitters including the optical films are described. In some cases, the optical film includes a first optical stack disposed on, and spaced apart by one or more spacer layers from, a second optical stack, each optical stack comprising a plurality of polymeric interference layers reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range. Each optical stack has interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths.

What is claimed is: 
     
         1 . An optical film comprising a plurality of stacked first polymeric interference layers disposed on a plurality of stacked second polymeric interference layers, each first and second interference layers reflecting or transmitting light primarily by optical interference for at least one wavelength in a same predetermined wavelength range extending at least from 450 to 600 nm, an outermost first interference layer being the first interference layer farthest from the plurality of stacked second interference layers, an outermost second interference layer being the second interference layer farthest from the plurality of stacked first interference layers, the outermost first and second interference layers having optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm. 
     
     
         2 . The optical film of  claim 1 , wherein for light in the predetermined wavelength range and substantially normally incident on the optical film, each of the plurality of stacked first polymeric interference layers and the plurality of stacked second polymeric interference layers has an average optical transmittance greater than about 80% for a first polarization state, an average optical reflectance greater than about 90% for an orthogonal second polarization state, and an average optical transmittance less than about 5% for the second polarization state. 
     
     
         3 . A polarizing beam splitter (PBS) comprising:
 a first prism comprising a first hypotenuse;   a second prism comprising a second hypotenuse facing the first hypotenuse; and   the optical film of  claim 1  disposed between and adhered to the first and second hypotenuses, the optical film further comprising a first optical stack facing the first prism and comprising the plurality of stacked first polymeric interference layers and a second optical stack facing the second prism and comprising the plurality of stacked second polymeric interference layers, the first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.   
     
     
         4 . The optical film of  claim 1 , wherein an innermost first interference layer is the first interference layer closest to the plurality of stacked second interference layers, and an innermost second interference layer is the second interference layer closest to the plurality of stacked first interference layers, the innermost first and second interference layers having optical thicknesses equal to one quarter of respective third and fourth wavelengths in the predetermined wavelength range, a difference between the third and fourth wavelengths being less than about 80 nm. 
     
     
         5 . The optical film of  claim 1  defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x-and y-axes, each of the plurality of stacked first polymeric interference layers and the plurality of stacked second interference layers comprising a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2. 
     
     
         6 . The optical film of  claim 5 , wherein for each first layer, 1.8≤nx≤1.9, 1.5≤ny≤1.6, and 1.5≤nz≤1.6, and for each second layer, each of nx, ny and nz is between 1.5 and 1.6.